There has hitherto been known an active matrix-type liquid crystal display device provided with a TFT (thin film transistor) as a switching element. This liquid crystal display device is provided with a liquid crystal panel configured of two insulating substrates opposed to each other. The one substrate of the liquid crystal panel is provided with gate bus lines (scanning signal lines) and source bus lines (video signal lines) in a matrix form, and is provided with TFTs in the vicinity of intersections between the gate bus lines and the source bus lines. Each TFT is configured of a gate electrode connected to the gate bus line, a source electrode connected to the source bus line, and a drain electrode. The drain electrode of each TFT is connected to one of a plurality of pixel electrodes that are arranged in a matrix form on the substrate so as to form an image. The other substrate of the liquid crystal panel is provided with a common electrode for applying a voltage between the common electrode and the pixel electrodes through a liquid crystal layer. In such a configuration, based on a video signal that the source electrode of each TFT receives from the source bus line when the gate electrode of this TFT receives an active scanning signal from the gate bus line, a voltage is applied between the pixel electrode and the common electrode. This drives the liquid crystal, and a desired image is displayed on a display portion of the liquid crystal panel.
Incidentally, the liquid crystal has a property that it deteriorates when a DC voltage continues to be applied. Accordingly, in the liquid crystal display device, AC driving to reverse polarities of pixel voltages (voltages between the pixel electrodes and the common electrode) has been performed in order to suppress deterioration in liquid crystal. As an AC driving technique, a driving technique called frame-reversal driving is known in which the polarities of the pixel voltages are reversed with respect to each frame in a state where the polarities of the pixel voltages in all the pixels are made the same. It is to be noted that the driving technique of reversing the polarities of the pixel voltages with respect to each predetermined period will be hereinafter referred to as a “reversal driving technique”. However, by the frame-reversal driving, flicker is relatively apt to occur at the time of image display. Hence there have hitherto been employed reversal driving techniques of a variety of polarity reversal patterns in order to suppress occurrence of flicker. As the reversal driving technique, column-reversal driving and dot-reversal driving are typically known.
The column-reversal driving is a driving technique of reversing the polarities of the pixel voltages with respect to each frame and each predetermined number of source bus lines. According to the column-reversal driving, the polarities of pixel voltages are reversed with respect to each predetermined number of source bus lines, and hence the frequency of spatial polarity reversal of a liquid crystal applied voltage becomes high as compared to the frame-reversal driving. For example, when the polarities of the pixel voltages are reversed with respect to each one frame and each one source bus line, polarities of pixel voltages in pixels on four rows and four columns in a certain frame become those as shown in FIG. 15. It is to be noted that in the next frame, the polarities of the pixel voltages are reversed in all the pixels.
The dot-reversal driving is a driving technique of reversing the polarities of the pixel voltages with respect to each one frame and also reversing the polarities in the pixels adjacent in a vertical or horizontal direction. In this driving technique, polarities of pixel voltages in pixels on four rows and four columns in a certain frame become those as shown in FIG. 16. It is to be noted that in the next frame, the polarities of the pixel voltages are reversed in all the pixels. According to this dot-reversal driving, the frequency of spatial polarity reversal of the liquid crystal applied voltage becomes still higher as compared to the column-reversal driving. That is, according to the dot-reversal driving, the polarity reversal pattern becomes complex as compared to the line-reversal driving and the column-reversal driving, thereby effectively suppressing occurrence of flicker. It should be noted that a driving technique of reversing the polarities of the pixel voltages with respect to each predetermined number of gate bus lines in the vertical direction is called “multi-dot-reversal driving”. For example, a driving technique of reversing the polarities of the pixel voltages with respect to each two gate bus lines in the vertical direction as shown in FIG. 17 is called “two-dot-reversal driving”.
In general, when a polarity reversal pattern in an employed reversal driving technique is complex, flicker hardly occurs, but power consumption becomes large. On the other hand, when a polarity reversal pattern in an employed reversal driving technique is simple, power consumption becomes small, but flicker is apt to occur. There has thus been required a technique for reducing power consumption while suppressing occurrence of flicker. For example, according to a liquid crystal display device disclosed in Japanese Patent Application Laid-Open No. 2005-215591, the dot-reversal driving and the column-reversal driving are switched in accordance with a frequency of an input video signal. Further, according to a liquid crystal display device disclosed in Japanese Patent Application Laid-Open No. 2003-337577, two-dot-reversal driving and one-dot-reversal driving are switched in accordance with a vertical frequency.